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Each of a lower reflective layer and an upper reflective layer are formed
at a corresponding one of the ends of an optical cavity in the thickness
direction. A main active layer is formed in the optical cavity between
the lower and upper reflective layers. The optical cavity includes an
auxiliary active layer in the vicinity of at least one of the lower
reflective layer and a second auxiliary active layer in the vicinity of
the upper reflective layer. The auxiliary active layer is located at
antinodes of a standing wave where the amplitude of light is large,
without increasing the physical length L or optical length Lo between the
lower reflective layer and the upper reflective layer.

Inventors:

TATE; Atsushi; (Kanagawa-ken, JP)

Assignee:

MURATA MANUFACTURING CO., LTD.Kyoto-fuJP

Serial No.:

950539

Series Code:

12

Filed:

November 19, 2010

Current U.S. Class:

372/45.01; 257/98; 257/E33.069

Class at Publication:

372/45.01; 257/98; 257/E33.069

International Class:

H01S 5/18 20060101 H01S005/18; H01L 33/10 20100101 H01L033/10

Foreign Application Data

Date

Code

Application Number

Nov 24, 2009

JP

2009-265989

Sep 8, 2010

JP

2010-200930

Claims

1. A surface emitting device comprising: an optical cavity including a
first active layer, a pair of reflective layers, the first active layer
being arranged between the pair of reflective layers, the reflective
layers being opposite each other, the optical cavity forming a standing
wave including antinodes where the maximum amplitude of light is
obtained, and each of the antinodes being located in the vicinity of a
corresponding one of the pair of reflective layers, a second active layer
formed in the vicinity of at least one of the pair of reflective layers,
and a cladding layer formed between the pair of reflective layers,
wherein the cladding layer is formed by a reduction method for reducing
the physical length or optical length of the cladding layer to offset an
increase in the physical length or optical length between the pair of
reflective layers due to the formation of the cladding layer and the
second active layer between the pair of reflective layers.

2. The surface emitting device according to claim 1, wherein the
reduction method includes forming the cladding layer such that the
physical length of the cladding layer offsets the increase in the
physical length or optical length between the pair of reflective layers.

3. The surface emitting device according to claim 1, wherein the
reduction method includes forming a film that serves as the cladding
layer of a low-refractive-index material such that the cladding layer has
an optical length that offsets said increase in the physical length or
optical length between the pair of reflective layers.

4. The surface emitting device according to claim 1, wherein the peak
wavelength of the gain spectrum of the second active layer is set to a
wavelength longer than the peak wavelength of the gain spectrum of the
first active layer, whereby a gain coefficient of the gain spectrum of
the second active layer shifted to shorter wavelengths due to a change in
operating temperature from room temperature to a low temperature is
superimposed on a frequency range where a gain coefficient of the gain
spectrum of the first active layer is reduced by the shift of the gain
spectrum of the first active layer to shorter wavelengths due to a change
in operating temperature from room temperature to a low temperature,
complementing a reduction in the gain coefficient of the first active
layer.

5. The surface emitting device according to claim 2, wherein the peak
wavelength of the gain spectrum of the second active layer is set to a
wavelength longer than the peak wavelength of the gain spectrum of the
first active layer, whereby a gain coefficient of the gain spectrum of
the second active layer shifted to shorter wavelengths due to a change in
operating temperature from room temperature to a low temperature is
superimposed on a frequency range where a gain coefficient of the gain
spectrum of the first active layer is reduced by the shift of the gain
spectrum of the first active layer to shorter wavelengths due to a change
in operating temperature from room temperature to a low temperature,
complementing a reduction in the gain coefficient of the first active
layer.

6. The surface emitting device according to claim 3, wherein the peak
wavelength of the gain spectrum of the second active layer is set to a
wavelength longer than the peak wavelength of the gain spectrum of the
first active layer, whereby a gain coefficient of the gain spectrum of
the second active layer shifted to shorter wavelengths due to a change in
operating temperature from room temperature to a low temperature is
superimposed on a frequency range where a gain coefficient of the gain
spectrum of the first active layer is reduced by the shift of the gain
spectrum of the first active layer to shorter wavelengths due to a change
in operating temperature from room temperature to a low temperature,
complementing a reduction in the gain coefficient of the first active
layer.

7. The surface emitting device according to claim 1, wherein the peak
wavelength of the gain spectrum of the second active layer is set to a
wavelength shorter than the peak wavelength of the gain spectrum of the
first active layer, whereby a gain coefficient of the gain spectrum of
the second active layer shifted to longer wavelengths due to a change in
operating temperature from room temperature to a high temperature is
superimposed on a frequency range where a gain coefficient of the gain
spectrum of the first active layer is reduced by the shift of the gain
spectrum of the first active layer to longer wavelengths due to a change
in operating temperature from room temperature to a high temperature,
complementing a reduction in the gain coefficient of the first active
layer.

8. The surface emitting device according to claim 2, wherein the peak
wavelength of the gain spectrum of the second active layer is set to a
wavelength shorter than the peak wavelength of the gain spectrum of the
first active layer, whereby a gain coefficient of the gain spectrum of
the second active layer shifted to longer wavelengths due to a change in
operating temperature from room temperature to a high temperature is
superimposed on a frequency range where a gain coefficient of the gain
spectrum of the first active layer is reduced by the shift of the gain
spectrum of the first active layer to longer wavelengths due to a change
in operating temperature from room temperature to a high temperature,
complementing a reduction in the gain coefficient of the first active
layer.

9. The surface emitting device according to claim 3, wherein the peak
wavelength of the gain spectrum of the second active layer is set to a
wavelength shorter than the peak wavelength of the gain spectrum of the
first active layer, whereby a gain coefficient of the gain spectrum of
the second active layer shifted to longer wavelengths due to a change in
operating temperature from room temperature to a high temperature is
superimposed on a frequency range where a gain coefficient of the gain
spectrum of the first active layer is reduced by the shift of the gain
spectrum of the first active layer to longer wavelengths due to a change
in operating temperature from room temperature to a high temperature,
complementing a reduction in the gain coefficient of the first active
layer.

10. The surface emitting device according to claim 1, wherein the
standing wave has a length corresponding to one wavelength of the light.

11. The surface emitting device according to claim 1, wherein said
cladding layer is a first of a pair of first and second cladding layers,
and said second cladding layer has a physical or optical length different
from said first cladding layer.

12. A surface emitting device comprising: an optical cavity including a
first active layer, a pair of reflective layers, the first active layer
being arranged between the pair of reflective layers, the reflective
layers being opposite each other, the optical cavity forming a standing
wave including antinodes where the maximum amplitude of light is
obtained, and each of the antinodes being located in the vicinity of a
corresponding one of the pair of reflective layers, and a second active
layer formed in the vicinity of at least one of the pair of reflective
layers.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to Japanese Patent
Application No. 2009-265989 filed Nov. 24, 2009, and to Japanese Patent
Application No. 2010-200939 filed Sep. 8, 2010, the entire contents of
which are incorporated herein by reference in their entirety.

[0003] In general, a surface emitting device has a laminated structure in
which a lower reflective layer of a first conductivity type, a plurality
of barrier layers, a plurality of active layers each having a
single-quantum-well structure or a multiple-quantum-well structure formed
between the barrier layers, and an upper reflective layer of a second
conductivity type are stacked, in that order, on the upper surface of a
substrate of the first conductivity type. A current confinement layer
configured to efficiently inject a current into the active layers is
formed on one of the lower reflective layer and the upper reflective
layer. Thereby, the structure including the lower reflective layer, the
plural barrier layers, the active layers, and the upper reflective layer
functions as an optical cavity. The lower reflective layer and the upper
reflective layer included in the optical cavity are arranged in such a
manner that the optical length therebetween is approximately "one-half
wavelength of light (.lamda./2).times.(1+n) (where n represents a natural
number)."

[0004] A first electrode is formed on the lower surface of the substrate.
A second electrode is formed on the upper surface of the upper reflective
layer. A voltage is applied between the first electrode and the second
electrode. The application of a voltage injects a current into the active
layers, emitting light from the active layers by stimulated emission. The
resulting light is repeatedly reflected between the lower reflective
layer and the upper reflective layer, forming the standing wave of light
(standing light wave). Each of the active layers is located in the middle
portion of the optical cavity and at a corresponding one of antinodes
where the maximum amplitude of light is obtained. This increases the
electric field of the standing wave propagating through the active layers
to enhance the optical output power from each of the active layers,
thereby improving the total optical output power. To further improve the
optical output power, the number of antinodes of the standing wave is
increased by increasing the length of the optical cavity, and an
additional active layer is formed at newly formed antinodes of the
standing wave (see, for example, Japanese Unexamined Patent Application
Publication Nos. 10-27945 and 2007-87994).

[0005] However, in the case where the additional active layer is formed at
the newly formed antinode of the standing wave obtained by increasing the
length of the optical cavity, the increased optical length of the optical
cavity disadvantageously leads to weak optical confinement, failing to
achieve a sufficient mode gain.

[0006] This problem will be described in detail below. The strength of
optical confinement for an optical cavity is expressed as an optical
confinement factor. The optical confinement factor is defined as the
proportion of light confined in the active layer. The ratio (d/L) of the
total physical (effective) length d of the active layers to the physical
(effective) length L between a pair of reflective layers of the optical
cavity is used as an indicator. The mode gain is expressed as the product
of a gain coefficient obtained in the active layer and the optical
confinement factor and indicates the effective optical gain of the
optical cavity. Thus, a weaker optical confinement, i.e., a smaller
optical confinement factor, results in a smaller mode gain of a surface
emitting device.

SUMMARY

[0007] The invention is directed to a surface emitting device that has a
large optical confinement factor and an efficiently increased mode gain.

[0008] In an embodiment consistent with the claimed invention, a surface
emitting device includes an optical cavity including a first active
layer, a pair of reflective layers opposite each other, the first active
layer being arranged between the pair of reflective layers, a second
active layer formed in the vicinity of at least one of the pair of
reflective layers, and a cladding layer formed between the pair of
reflective layers. The optical cavity forms a standing wave including
antinodes where the maximum amplitude of light is obtained, and each of
the antinodes is located in the vicinity of a corresponding one of the
pair of reflective layers. The cladding layer is formed by a reduction
method for reducing the physical length or optical length of the cladding
layer to offset an increase in the physical length or optical length
between the pair of reflective layers due to the formation of the
cladding layer and the second active layer between the pair of reflective
layers.

[0009] Forming the second active layer in the vicinity of at least one of
the pair of reflective layers permits forming the standing wave including
antinodes where the maximum amplitude of light is obtained. Each of the
antinodes is located in the vicinity of a corresponding one of the pair
of reflective layers. Thus, the amplitude of light can be increased also
at the second active layer, which makes it possible to improve the
optical output power.

[0010] Furthermore, the second active layer is formed in the optical
cavity without increasing the optical length between the pair of
reflective layers. It is thus possible to suppress an increase in
physical length L between the pair of reflective layers as compared with
the case where the number of antinodes of the standing wave is increased
by increasing the optical length between the pair of reflective layers
and where the additional active layer is formed at the newly formed
antinode. This can efficiently increase the ratio (d/L), which serves as
an indicator of the optical confinement factor, and efficiently increase
in mode gain.

[0011] Moreover, the cladding layer is formed between the pair of
reflective layers by the reduction method for reducing the physical
length or optical length thereof. The reduction method enables offsetting
an increase in the physical length or optical length between the pair of
reflective layers due to the formation of the second active layer. It is
thus possible to keep the optical length constant while suppressing an
increase in the physical length between the pair of reflective layers
included in the optical cavity. This can result in an efficient increase
in the ratio (d/L), which serves as an indicator of the optical
confinement factor, and an efficient increase in mode gain.

[0012] According to a more specific exemplary embodiment, the reduction
method includes forming the cladding layer in such a manner that it has a
length that offsets an increase in the physical length or optical length
between the pair of reflective layers.

[0013] In this case, the cladding layer may be formed to have a short
physical length to compensate for the second active layer. This can
enable offsetting an increase in the physical length or optical length
between the pair of reflective layers due to the formation of the second
active layer. Thus the optical length between the pair of reflective
layers included in the optical cavity can be kept constant.

[0014] According to another more specific exemplary embodiment, the
reduction method may include forming a film that serves as the cladding
layer of a low-refractive-index material such that the cladding layer has
a short optical length that offsets said increase in the physical length
or optical length between the pair of reflective layers.

[0015] In this case, the optical length of the cladding layer may be
reduced in response to the second active layer, which enables offsetting
an increase in the physical length or optical length between the pair of
reflective layers due to the formation of the second active layer. Thus
the optical length between the pair of reflective layers included in the
optical cavity can be kept constant.

[0016] According to yet another more specific exemplary embodiment, the
peak wavelength of the gain spectrum of the second active layer may be
set to a wavelength longer than the peak wavelength of the gain spectrum
of the first active layer, whereby a gain coefficient of the gain
spectrum of the second active layer shifted to shorter wavelengths due to
a change in operating temperature from room temperature to a low
temperature is superimposed on a frequency range where a gain coefficient
of the gain spectrum of the first active layer is reduced by the shift of
the gain spectrum of the first active layer to shorter wavelengths due to
a change in operating temperature from room temperature to a low
temperature, complementing a reduction in the gain coefficient of the
first active layer.

[0017] In this case, the peak wavelength of the gain spectrum of the
second active layer may be set to a wavelength longer than the peak
wavelength of the gain spectrum of the first active layer. Thus, the gain
spectrum of the second active layer shifted to shorter wavelengths due to
a change in operating temperature from room temperature to a low
temperature may be superimposed on the gain spectrum of the first active
layer in a frequency range where the gain coefficient may be reduced by
the shift of the gain spectrum of the first active layer to shorter
wavelengths due to a change in operating temperature from room
temperature to a low temperature, thereby complementing a reduction in
the gain coefficient of the first active layer due to a change in
operating temperature from room temperature to a low temperature. This
can lead to stable operating characteristics even if the operating
temperature is changed from room temperature to a low temperature.

[0018] According to another more specific exemplary embodiment, the peak
wavelength of the gain spectrum of the second active layer may be set to
a wavelength shorter than the peak wavelength of the gain spectrum of the
first active layer, whereby a gain coefficient of the gain spectrum of
the second active layer shifted to longer wavelengths due to a change in
operating temperature from room temperature to a high temperature may be
superimposed on a frequency range where a gain coefficient of the gain
spectrum of the first active layer is reduced by the shift of the gain
spectrum of the first active layer to longer wavelengths due to a change
in operating temperature from room temperature to a high temperature,
complementing a reduction in the gain coefficient of the first active
layer.

[0019] In this case, the peak wavelength of the gain spectrum of the
second active layer may be set to a wavelength shorter than the peak
wavelength of the gain spectrum of the first active layer. Thus, the gain
spectrum of the second active layer shifted to longer wavelengths due to
a change in operating temperature from room temperature to a high
temperature may be superimposed on the gain spectrum of the first active
layer in a frequency range where the gain coefficient is reduced by the
shift of the gain spectrum of the first active layer to longer
wavelengths due to a change in operating temperature from room
temperature to a high temperature, thereby complementing a reduction in
the gain coefficient of the first active layer due to a change in
operating temperature from room temperature to a high temperature. This
can lead to stable operating characteristics even if the operating
temperature is changed from room temperature to a high temperature.

[0020] In another embodiment consistent with the claimed invention, a
surface emitting device includes an optical cavity including a first
active layer, a pair of reflective layers opposite each other, and a
second active layer formed in the vicinity of at least one of the pair of
reflective layers. The first active layer is arranged between the pair of
reflective layers, and the optical cavity forms a standing wave including
antinodes where the maximum amplitude of light is obtained. Each of the
antinodes is located in the vicinity of a corresponding one of the pair
of reflective layers.

[0021] In this case, the second active layer makes it possible to improve
the optical output power by increasing the amplitude of light at the
second active layer in the same way as described above. Furthermore, it
is possible to suppress an increase in physical length L between the pair
of reflective layers. This can efficiently increase in the ratio (d/L),
which serves as an indicator of the optical confinement factor, and
efficiently increase mode gain.

[0022] Other features, elements, characteristics and advantages of the
present invention will become more apparent from the following detailed
description of preferred embodiments of the present invention with
reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 is a cross-sectional view of a vertical-cavity
surface-emitting laser device according to a first exemplary embodiment.

[0024] FIG. 2 is a cross-sectional view of a vertical-cavity
surface-emitting laser device according to a first comparative
embodiment.

[0025] FIG. 3 is a cross-sectional view of a vertical-cavity
surface-emitting laser device according to a second comparative
embodiment.

[0026] FIG. 4 is a cross-sectional view of a vertical-cavity
surface-emitting laser device according to a second exemplary embodiment.

[0027] FIG. 5 depicts characteristic curves indicating the gain spectra of
a common vertical-cavity surface-emitting laser device at room
temperature, a low temperature, and a high temperature.

[0028] FIG. 6 depicts characteristic curves indicating the gain spectrum
of the vertical-cavity surface-emitting laser device illustrated in FIG.
4 and the gain spectrum of a main active layer alone at room temperature.

[0029] FIG. 7 depicts characteristic curves indicating the gain spectrum
of the vertical-cavity surface-emitting laser device illustrated in FIG.
4 and the gain spectrum of the main active layer alone at a low
temperature.

[0030] FIG. 8 is a cross-sectional view of a vertical-cavity
surface-emitting laser device according to a third exemplary embodiment.

[0031] FIG. 9 depicts characteristic curves indicating the gain spectrum
of the vertical-cavity surface-emitting laser device illustrated in FIG.
8 and the gain spectrum of a main active layer alone at room temperature.

[0032] FIG. 10 depicts characteristic curves indicating the gain spectrum
of the vertical-cavity surface-emitting laser device illustrated in FIG.
8 and the gain spectrum of the main active layer alone at a high
temperature.

DETAILED DESCRIPTION

[0033] Exemplary embodiments of the present invention will be described in
detail below using exemplary vertical-cavity surface-emitting laser
devices (hereinafter, referred to as "VCSELs") with reference to the
attached drawings.

[0034] FIG. 1 shows a VCSEL 1 according to a first exemplary embodiment.
VCSEL 1 has a laminated structure in which a lower reflective layer 4, a
first barrier layer 9, a first auxiliary active layer 7 (second active
layer), a first cladding layer 11, a main active layer 6 (first active
layer), a second cladding layer 12, a second auxiliary active layer 8
(second active layer), a second barrier layer 10, and an upper reflective
layer 5 are sequentially stacked on the upper surface of a substrate 2.
An optical cavity 3 is formed of the lower reflective layer 4, the first
barrier layer 9, the first auxiliary active layer 7, the first cladding
layer 11, the main active layer 6, the second cladding layer 12, the
second auxiliary active layer 8, the second barrier layer 10, and the
upper reflective layer 5. Although not shown, the upper reflective layer
5 is provided with a current confinement layer. Furthermore, a contact
layer (not shown) is provided on the uppermost portion of the upper
reflective layer 5 in order to form an ohmic contact with a p-type
electrode 14 described below.

[0035] An n-type electrode 13 is formed on the lower surface of the
substrate 2, and a p-type electrode 14 is formed on the upper surface of
the upper reflective layer 5. The lower reflective layer 4, the first
barrier layer 9, the first auxiliary active layer 7, the first cladding
layer 11, the main active layer 6, the second cladding layer 12, the
second auxiliary active layer 8, the second barrier layer 10, and the
upper reflective layer 5 can be formed by an epitaxial growth technique
such as metal-organic chemical vapor deposition (MOCVD). The n-type
electrode 13 and the p-type electrode 14 are formed of conductive thin
metal films and formed by, for example, evaporation or sputtering. An
opening 14A configured to emit light is formed in the central portion of
the p-type electrode 14.

[0036] The substrate 2 is formed of, for example, a substrate having a
thickness of about several hundred micrometers and being composed of a
compound semiconductor of n-type single-crystal gallium arsenide
(n-GaAs).

[0037] The lower reflective layer 4 is constituted by an n-type
distributed Bragg reflector (DBR) in which a plurality of thin layers
composed of an n-type compound semiconductor of n-type single-crystal
aluminum gallium arsenide (n-Al.sub.0.12Ga.sub.0.88As) having an Al
content of, for example, about 12% and a plurality of thin layers
composed of an n-type compound semiconductor of n-type single-crystal
aluminum gallium arsenide (n-Al.sub.0.9Ga.sub.0.1As) having an Al content
of, for example, about 90% are alternately stacked (for example, about 30
layers each). The optical length of each of the thin layers composed of
single-crystal n-Al.sub.0.12Ga.sub.0.88As and the thin layers composed of
single-crystal n-Al.sub.0.9Ga.sub.0.1As is set to about .lamda./4 with
respect to the wavelength .lamda. (e.g., .lamda.=about 850 nm) of light
generated in the optical cavity 3. Note that the optical length is
calculated by multiplying the physical (effective) length of the
thickness by the refractive index of the layer (medium).

[0038] Like the lower reflective layer 4, the upper reflective layer 5 is
constituted by a p-type distributed Bragg reflector in which a plurality
of thin layers composed of a p-type compound semiconductor of p-type
single-crystal aluminum gallium arsenide (p-Al.sub.0.12Ga.sub.0.88As)
having an Al content of, for example, about 12% and a plurality of thin
layers composed of a p-type compound semiconductor of p-type
single-crystal aluminum gallium arsenide (p-Al.sub.0.9Ga.sub.0.1As)
having an Al content of, for example, about 90% are alternately stacked
(for example, about 10 layers each). The optical length of each of the
thin layers composed of single-crystal p-Al.sub.0.12Ga.sub.0.88As and the
thin layers composed of single-crystal p-Al.sub.0.9Ga.sub.0.1As is set to
about .lamda./4 with respect to the wavelength .lamda. (e.g.,
.lamda.=about 850 nm) of light generated in the optical cavity 3.

[0039] The lower reflective layer 4 and the upper reflective layer 5
included in the optical cavity 3 are spaced in such a manner that the
optical length Lo between the lower reflective layer 4 and the upper
reflective layer 5 is comparable to, for example, the wavelength .lamda.
(about 850 nm) of light.

[0040] The main active layer 6 is arranged in the middle portion of the
optical cavity 3 in the thickness direction. The main active layer 6 has
a multiple-quantum-well structure that includes, for example, three well
sublayers 6A, 6B, and 6C functioning as quantum wells. Each of the well
sublayers 6A, 6B, and 6C is composed of, for example, single-crystal
gallium arsenide (GaAs) and has a thickness of about several nanometers.
A barrier layer 6D is formed between the well layers 6A and 6B. A barrier
layer 6E is formed between the well sublayers 6B and 6C. Each of the
barrier layers 6D and 6E is composed of, for example, single-crystal
aluminum gallium arsenide (Al.sub.0.3Ga.sub.0.7As) having an aluminum
content of about 30% and has a thickness of about several nanometers.

[0041] The first auxiliary active layer 7 is arranged in the vicinity of
the lower reflective layer 4 with the first barrier layer 9. The first
auxiliary active layer 7 has a single-quantum-well structure. Like the
well sublayers 6A, 6B, and 6C of the main active layer 6, the first
auxiliary active layer 7 is composed of, for example, single-crystal
gallium arsenide (GaAs) and has a thickness of about several nanometers.
The first barrier layer 9 is composed of, for example, single-crystal
aluminum gallium arsenide (Al.sub.0.3Ga.sub.0.7As) having an aluminum
content of about 30%.

[0042] The second auxiliary active layer 8 is arranged in the vicinity of
the upper reflective layer 5 with the second barrier layer 10. The second
auxiliary active layer 8 has a single-quantum-well structure. Like the
first auxiliary active layer 7, the second auxiliary active layer 8 is
composed of, for example, single-crystal gallium arsenide (GaAs) and has
a thickness of about several nanometers. The second barrier layer 10 is
composed of, for example, single-crystal aluminum gallium arsenide
(Al.sub.0.3Ga.sub.0.7As) having an aluminum content of about 30%. The
peak wavelength of the gain spectrum of each of the first auxiliary
active layer 7 and the second auxiliary active layer 8 is set to
substantially the same value as that of the main active layer 6.

[0043] As described below, antinodes where the maximum amplitude of light
in a standing wave S is obtained are located at the first auxiliary
active layer 7, the main active layer 6, and the second auxiliary active
layer 8. Note that the maximum intensity of light is obtained at the
antinodes where the maximum amplitude of light is obtained. Thus, the
arrangement of the first auxiliary active layer 7 to a position as close
to the lower reflective layer 4 as possible improves the optical output
power from the first auxiliary active layer 7. Similarly, the arrangement
of the second auxiliary active layer 8 to a position as close to the
upper reflective layer 5 as possible improves the optical output power
from the second auxiliary active layer 8.

[0044] However, an excessively short physical (effective) length of each
of the first barrier layer 9 and the second barrier layer 10 possibly
results in excessively short distances between the first auxiliary active
layer 7 and the lower reflective layer 4 and between the second auxiliary
active layer 8 and the upper reflective layer 5, reducing the film
quality of the first auxiliary active layer 7 and the second auxiliary
active layer 8. Thus, the thickness of the first barrier layer 9 is
preferably set to a minimum possible value to the extent that a
sufficient distance between the first auxiliary active layer 7 and the
lower reflective layer 4 is obtained. Usually, the thickness of the first
barrier layer 9 is set to a minimum possible value such as about 5 nm.

[0045] Like the first barrier layer 9, the thickness of the second barrier
layer 10 is preferably set to a minimum possible value to the extent that
a sufficient distance between the second auxiliary active layer 8 and the
upper reflective layer 5 is obtained. Usually, the thickness of the
second barrier layer 10 is set to a minimum possible value such as about
5 nm.

[0046] The first cladding layer 11 is arranged between the first auxiliary
active layer 7 and the main active layer 6. The second cladding layer 12
is arranged between the main active layer 6 and the second auxiliary
active layer 8. Each of the first cladding layer 11 and the second
cladding layer 12 is composed of, for example, single-crystal aluminum
gallium arsenide (Al.sub.0.3Ga.sub.0.7As) having an aluminum content of
about 30%.

[0047] The total optical length of the first cladding layer 11 and the
second cladding layer 12 is determined by subtracting the optical lengths
of the main active layer 6, the first auxiliary active layer 7, the
second auxiliary active layer 8, the first barrier layer 9, and the
second barrier layer 10 from the optical length Lo between the lower
reflective layer 4 and the upper reflective layer 5.

[0048] Specifically, the optical length Lo between the lower reflective
layer 4 and the upper reflective layer 5 is set to, for example, a value
comparable to the wavelength .lamda. of light (about 850 nm). The optical
length of the main active layer 6 is set to about 160 nm. The optical
length of each of the first auxiliary active layer 7 and the second
auxiliary active layer 8 is set to about 30.8 nm. The optical length of
each of the first barrier layer 9 and the second barrier layer 10 is set
to about 33.8 nm. In this case, the total optical length of the first
cladding layer 11 and the second cladding layer 12 is set to about 560.4
nm.

[0049] The first cladding layer 11 and the second cladding layer 12 are
usually formed so as to have the same optical length, so that each of the
first cladding layer 11 and the second cladding layer 12 has an optical
length of about 280.4 nm. Thus, the physical (effective) lengths of the
first cladding layer 11 and the second cladding layer 12 in the thickness
direction are also reduced by values corresponding to, for example, the
respective first auxiliary active layer 7 and the second auxiliary active
layer 8. In this way, the optical lengths of the first cladding layer 11
and the second cladding layer 12 are adjusted by a reduction method to
offset the optical lengths of the first auxiliary active layer 7 and the
second auxiliary active layer 8, so that the optical length Lo between
the lower reflective layer 4 and the upper reflective layer 5 included in
the optical cavity 3 is kept constant.

[0050] Furthermore, the first cladding layer 11 and the second cladding
layer 12 increase the electron and hole densities of the main active
layer 6, the first auxiliary active layer 7, and the second auxiliary
active layer 8 and function to confine light to the main active layer 6,
the first auxiliary active layer 7, and the second auxiliary active layer
8 in the same way as in the first barrier layer 9 and the second barrier
layer 10.

[0051] The VCSEL 1 according to this embodiment has the foregoing
structure. The operation of the VCSEL 1 will be described below.

[0052] The application of a voltage between the n-type electrode 13 and
the p-type electrode 14 injects a current into the first auxiliary active
layer 7, the main active layer 6, and the second auxiliary active layer
8, thus exciting the well sublayers 6A to 6C and so forth included in
these layers and causing stimulated emission of light. The resulting
light is repeatedly reflected between the lower reflective layer 4 and
the upper reflective layer 5, forming the standing wave S between the
lower reflective layer 4 and the upper reflective layer 5 and emitting
light having a wavelength of .lamda. through the opening 14A. The
standing wave S with a length corresponding to one wavelength .lamda. of
light is formed in the optical cavity 3. An antinode where the maximum
amplitude of the standing wave S is obtained is located at each of the
main active layer 6, first auxiliary active layer 7, and the second
auxiliary active layer 8.

[0053] In the standing wave S, the maximum light intensity is obtained at
the antinodes where the maximum amplitude of light is obtained. That is,
the maximum intensity of the standing wave S is obtained at the main
active layer 6, the first auxiliary active layer 7, and the second
auxiliary active layer 8. Thus, the first auxiliary active layer 7 and
the second auxiliary active layer 8 make it possible to improve the
optical output power.

[0054] The optical confinement factor of the VCSEL 1 according to the
first exemplary embodiment will be described below in comparison with the
optical confinement factor of a VCSEL 101 shown in FIG. 2, which does not
include the first auxiliary active layer 7 and the second auxiliary
active layer 8, according to a first comparative embodiment. Note that
the optical length of the optical cavity of the VCSEL 101 is equal to
that of the VCSEL 1.

[0055] As shown in FIG. 2, VCSEL 101 according to the first comparative
embodiment includes an optical cavity 103 on the upper surface of a
substrate 102. The optical cavity 103 includes a lower reflective layer
104, a first cladding layer 107, an active layer 106, a second cladding
layer 108, and an upper reflective layer 105 stacked in that order. An
n-type electrode 109 is formed on the substrate 102. A p-type electrode
110 having an opening 110A is formed on the upper surface of the upper
reflective layer 105.

[0056] As with the VCSEL 1 according to the first exemplary embodiment,
also in the case of the VCSEL 101, the optical length Lo of the optical
cavity 103 is set to a value comparable to the wavelength .lamda. of
light. For the VCSEL 101 according to the first comparative embodiment,
the active layer 106 including, for example, three well sublayers 106A,
106B, and 106C is formed at an antinode of the standing wave S. A barrier
layer 106D is formed between the well sublayers 106A and 106B. A barrier
layer 106E is formed between the well sublayers 106B and 106C.

[0057] For the first comparative embodiment, however, the total physical
length d of the active layer 106 is reduced by the thicknesses of the
first and second auxiliary active layers 7 and 8, as compared with that
in the first embodiment. Thus, the ratio (d/L) serving as an indicator of
an optical confinement factor is reduced compared with that in the first
embodiment. Specifically, the ratio (d/L) in the VCSEL 1 is about 0.059,
whereas the ratio (d/L) in the VCSEL 101 is about 0.034.

[0058] The optical confinement factor of the VCSEL 1 according to the
first exemplary embodiment will be described below in comparison with the
optical confinement factor of a VCSEL 121 according to a second
comparative embodiment in which the number of antinodes of the standing
wave is increased by increasing the optical length of the optical cavity
and in which an active layer is further formed at the newly formed
antinode.

[0059] FIG. 3 illustrates the VCSEL 121 according to the second
comparative embodiment. The VCSEL 121 includes an optical cavity 123 on
the upper surface of a substrate 122. The optical cavity 123 includes a
lower reflective layer 124, a first cladding layer 128, an active layer
126, a second cladding layer 129, an active layer 127, a third cladding
layer 130, and an upper reflective layer 125 stacked in that order. An
n-type electrode 131 is formed on the lower surface of the substrate 122.
A p-type electrode 132 having an opening 132A is formed on the upper
surface of the upper reflective layer 125.

[0060] The optical length of the optical cavity 3 of the VCSEL 1 according
to the first embodiment is set to a value equal to the wavelength .lamda.
of light, whereas the optical length Lc of the optical cavity 123 of the
VCSEL 121 according to the second comparative embodiment is set to, for
example, a value about 1.5 times the wavelength .lamda. of light. In this
case, a standing wave S having a length about 1.5 times the wavelength
.lamda. of light is formed in the optical cavity 123 of the VCSEL 121. A
new antinode where the maximum amplitude is obtained is formed in the
middle portion of the standing wave S.

[0061] Thus, in the VCSEL 121 according to the second comparative
embodiment, two antinodes are located in the middle portion of the
standing wave S. The active layer 126 including, for example, three well
sublayers 126A, 126B, and 126C is formed at a portion where one of the
two antinodes is located close to the lower reflective layer 124. A
barrier layer 126D is formed between the well sublayers 126A and 126B. A
barrier layer 126E is formed between the well sublayers 126B and 126C.

[0062] In addition, the active layer 127 including two well sublayers 127A
and 127B is formed at a position corresponding to the new antinode, i.e.,
at a portion where one of the two antinodes, which are located in the
middle portion of the standing wave S, is located close to the upper
reflective layer 125. A barrier layer 127C is formed between the well
sublayers 127A and 127B. The cladding layer 129 is formed between the
active layers 126 and 127. Thus, the total physical (effective) length of
the active layers included in the optical cavity 123 between the lower
reflective layer 124 and the upper reflective layer 125 is increased by
the thicknesses of the well sublayers 127A and 127B compared with the
case where only the active layer 126 is formed.

[0063] However, the physical (effective) length L' between the lower
reflective layer 124 and the upper reflective layer 125 of the optical
cavity 123 according to the second comparative embodiment is as large as
about 1.5 times the physical (effective) length L between the lower
reflective layer 4 and the upper reflective layer 5 of the optical cavity
3 according to the first exemplary embodiment. Thus, the ratio (d/L')
serving as an indicator of an optical confinement factor is not
increased. Therefore, formation of the active layer 127 is ineffective.
Specifically, the ratio (d/L) in the VCSEL 1 is about 0.059, whereas the
ratio (d/L') in the VCSEL 121 is about 0.054.

[0064] Thus, the mode gain is efficiently increased in the VCSEL 1
according to the first exemplary embodiment compared with the VCSELs 101
and 121 according to the first and second comparative embodiments.

[0065] As described above, according to the first exemplary embodiment,
each of the auxiliary active layers 7 and 8 is formed in the vicinity of
a corresponding one of the lower reflective layer 4 and the upper
reflective layer 5, which are located at the antinodes of the standing
wave S, thereby increasing the amplitude of light also at the auxiliary
active layers 7 and 8. That is, the auxiliary active layers 7 and 8 make
it possible to improve the optical output power.

[0066] Furthermore, the auxiliary active layers 7 and 8 are formed in the
optical cavity 3 without increasing the optical length Lo between the
lower reflective layer 4 and the upper reflective layer 5. It is thus
possible to suppress an increase in physical length L between the lower
reflective layer 4 and the upper reflective layer 5 as compared with the
VCSEL 121 according to the second comparative embodiment in which the
number of antinodes of the standing wave S is increased by increasing the
optical length Lc between the lower reflective layer 124 and the upper
reflective layer 125 and in which the additional active layer 127 is
formed at the newly formed antinode. This can therefore result in an
efficient increase in ratio (d/L), which serves as an indicator of the
optical confinement factor, and an efficient increase in mode gain.

[0067] In addition, the active layer 127 including two well sublayers 127A
and 127B is formed at a position corresponding to the new antinode, i.e.,
at a portion where one of the two antinodes, which are located in the
middle portion of the standing wave S, is located close to the upper
reflective layer 125. A barrier layer 127C is formed between the well
sublayers 127A and 127B. The cladding layer 129 is formed between the
active layers 126 and 127. Thus, the total physical (effective) length of
the active layers included in the optical cavity 123 between the lower
reflective layer 124 and the upper reflective layer 125 is increased by
the thicknesses of the well sublayers 127A and 127B compared with the
case where only the active layer 126 is formed.

[0068] However, the physical (effective) length L' between the lower
reflective layer 124 and the upper reflective layer 125 of the optical
cavity 123 according to the second comparative embodiment is as large as
about 1.5 times the physical (effective) length L between the lower
reflective layer 4 and the upper reflective layer 5 of the optical cavity
3 according to the first embodiment. Thus, the ratio (d/L') serving as an
indicator of an optical confinement factor is not increased. Hence, the
formation of the active layer 127 is ineffective. Specifically, the ratio
(d/L) in the VCSEL 1 is about 0.059, whereas the ratio (d/L') in the
VCSEL 121 is about 0.054. Thus, the mode gain is efficiently increased in
the VCSEL 1 according to the first exemplary embodiment compared with the
VCSEL 101, 121 according to the first or second comparative embodiments.

[0069] As described above, according to the first exemplary embodiment,
each of the auxiliary active layers 7 and 8 is formed in the vicinity of
a corresponding one of the lower reflective layer 4 and the upper
reflective layer 5, which are located at the antinodes of the standing
wave S, thereby increasing the amplitude of light also at the auxiliary
active layers 7 and 8. That is, the auxiliary active layers 7 and 8 make
it possible to improve the optical output power.

[0070] Furthermore, the auxiliary active layers 7 and 8 are formed in the
optical cavity 3 without increasing the optical length Lo between the
lower reflective layer 4 and the upper reflective layer 5. It is thus
possible to suppress an increase in physical length L between the lower
reflective layer 4 and the upper reflective layer 5 as compared with the
VCSEL 121 according to the second comparative embodiment in which the
number of antinodes of the standing wave S is increased by increasing the
optical length Lc between the lower reflective layer 124 and the upper
reflective layer 125 and in which the additional active layer 127 is
formed at the newly formed antinode. This can therefore result in an
efficient increase in ratio (d/L), which serves as an indicator of the
optical confinement factor, and an efficient increase in mode gain.

[0071] In addition, the cladding layers 11 and 12 are formed between the
lower reflective layer 4 and the upper reflective layer 5 by a reduction
method for reducing the physical lengths of the cladding layers 11 and
12. The reduction method enables offsetting an increase in the physical
length L or optical length Lo between the lower reflective layer 4 and
the upper reflective layer 5 due to the formation of the auxiliary active
layers 7 and 8. It is thus possible to keep the optical length Lo
constant while suppressing an increase in the physical length L between
the lower reflective layer 4 and the upper reflective layer 5 included in
the optical cavity 3. This therefore results in an efficient increase in
ratio (d/L), which serves as an indicator of the optical confinement
factor, and an efficient increase in mode gain.

[0072] In the first exemplary embodiment, the reduction method includes
forming the first cladding layer 11 and the second cladding layer 12 in
such a manner that each of the first cladding layer 11 and the second
cladding layer 12 has a short physical (effective) length. However, the
present invention is not limited to this reduction method. As another
reduction method, for example, the first cladding layer 11 and the second
cladding layer 12 can be formed, without changing their physical
(effective) lengths, using a material having a lower refractive index
than that of the cladding layer used when the first auxiliary active
layer 7 and the second auxiliary active layer 8 are not formed. In this
case, since the optical length of each of the first cladding layer 11 and
the second cladding layer 12 is given by the product of the physical
(effective) length of the thickness and the refractive index of the layer
(medium), each of the first cladding layer 11 and the second cladding
layer 12 has a short optical length. Hence, another reduction method
provides the same effect as in the case where the physical (effective)
length is changed.

[0073] Instead of changing the physical (effective) lengths of the first
cladding layer 11 and the second cladding layer 12, the physical
(effective) lengths of the first barrier layer 9 and the second barrier
layer 10 can be changed. However, excessively short physical (effective)
lengths of the first barrier layer 9 and the second barrier layer 10 are
likely to cause a reduction in the quality of the first auxiliary active
layer 7 and the second auxiliary active layer 8. It is thus desirable to
change the physical (effective) lengths of the first cladding layer 11
and the second cladding layer 12.

[0074] Moreover, in the first exemplary embodiment, the first auxiliary
active layer 7 and the second auxiliary active layer 8 are formed at
respective positions below and above the main active layer 6.
Alternatively, only one of the first auxiliary active layer 7 and the
second auxiliary active layer 8 can be formed.

[0075] In addition, in the first embodiment, each of the first auxiliary
active layer 7 and the second auxiliary active layer 8 has a single
quantum well structure. Alternatively, each of them can have a
multiple-quantum-well structure.

[0076] A second exemplary embodiment will be described below with
reference to FIG. 4. A first feature of the second exemplary embodiment
is the formation of an auxiliary active layer located close to one of an
upper reflective layer and a lower reflective layer or is the formation
of auxiliary active layers located close to both of an upper reflective
layer and a lower reflective layer included in an optical cavity of a
VCSEL. A second feature is the fact that the peak wavelength of the gain
spectrum of the auxiliary active layer is set to a wavelength longer than
the peak wavelength of the gain spectrum of a main active layer in order
to improve operation at low temperatures. In the second exemplary
embodiment illustrated in FIG. 4, descriptions will be made by taking the
case where the auxiliary reflective layer is formed close to the lower
reflective layer as an example.

[0077] A VCSEL 41 according to the second exemplary embodiment includes an
optical cavity 43 on the upper surface of a substrate 42. The optical
cavity 43 includes a lower reflective layer 44, a barrier layer 48, an
auxiliary active layer 47 (second active layer), a first cladding layer
49, a main active layer 46 (first active layer), a second cladding layer
50, and an upper reflective layer 45 stacked in that order. An n-type
electrode 51 is formed on the lower surface of the substrate 42. A p-type
electrode 52 having an opening 52A is formed on the upper surface of the
upper reflective layer 45.

[0078] The substrate 42, the lower reflective layer 44, and the upper
reflective layer 45 according to the second exemplary embodiment are
formed in substantially the same way as the substrate 2, the lower
reflective layer 4, and the upper reflective layer 5, respectively,
according to the first exemplary embodiment.

[0079] The lower reflective layer 44 and the upper reflective layer 45
included in the optical cavity 43 are spaced in such a manner that the
optical length Lo between the lower reflective layer 44 and the upper
reflective layer 45 is comparable to, for example, the wavelength .lamda.
(about 850 nm) of light.

[0080] The main active layer 46 is arranged in the middle portion of the
optical cavity 43 in the thickness direction. The main active layer 46
has a multiple-quantum-well structure that includes, for example, three
well sublayers 46A, 46B, and 46C functioning as quantum wells. Each of
the well sublayers 46A, 46B, and 46C is composed of, for example,
single-crystal gallium arsenide (GaAs) and has a thickness of about
several nanometers. A barrier layer 46D is formed between the well layers
46A and 46B. A barrier layer 46E is formed between the well sublayers 46B
and 46C. Each of the barrier layers 46D and 46E is composed of, for
example, single-crystal aluminum gallium arsenide
(Al.sub.0.3Ga.sub.0.7As) having an aluminum content of about 30% and has
a thickness of about several nanometers. The peak wavelength of the gain
spectrum of the main active layer 46 is set to, for example, about 835
nm.

[0081] The auxiliary active layer 47 is arranged in the vicinity of the
lower reflective layer 44 with the barrier layer 48. The auxiliary active
layer 47 has a single-quantum-well structure. Like the well sublayers
46A, 46B, and 46C of the main active layer 46, the auxiliary active layer
47 is composed of, for example, single-crystal gallium arsenide (GaAs)
and has a thickness of about several nanometers. The barrier layer 48 is
composed of, for example, single-crystal aluminum gallium arsenide
(Al.sub.0.3Ga.sub.0.7As) having an aluminum content of about 30%. The
peak wavelength of the gain spectrum of the auxiliary active layer 47 is
set to, for example, about 848 nm so as to be longer than the peak
wavelength of the gain spectrum of the main active layer 46.

[0082] A standing wave S with a length corresponding to one wavelength of
light having a wavelength of .lamda. is formed in the optical cavity 43.
In this case, the main active layer 46 and the auxiliary active layer 47
are located at antinodes where the maximum amplitude of light in the
standing wave S is obtained.

[0083] The first cladding layer 49 is arranged between the auxiliary
active layer 47 and the main active layer 46. The second cladding layer
50 is arranged between the main active layer 46 and the upper reflective
layer 45. Each of the first cladding layer 49 and the second cladding
layer 50 is composed of, for example, single-crystal aluminum gallium
arsenide (Al.sub.0.3Ga.sub.0.7As) having an aluminum content of about
30%.

[0084] The total optical length of the first cladding layer 49 and the
second cladding layer 50 is determined by subtracting the optical lengths
of the main active layer 46, the auxiliary active layer 47, and the
barrier layer 48 from the optical length Lo between the lower reflective
layer 44 and the upper reflective layer 45. The main active layer 46 is
arranged in the middle portion of the optical cavity 43 in the thickness
direction. Thus, for example, the total optical length of the first
cladding layer 49, the auxiliary active layer 47, and the barrier layer
48 is set to a value substantially equal to the optical length of the
second cladding layer 50.

[0085] Hence, the physical (effective) length of the first cladding layer
49 in the thickness direction is reduced by a value corresponding to, for
example, the auxiliary active layer 47 with respect to the physical
(effective) length of the second cladding layer 50. In this way, the
optical length of the first cladding layer 49 is adjusted by a reduction
method to offset the optical length of the auxiliary active layer 47, so
that the optical length Lo between the lower reflective layer 44 and the
upper reflective layer 45 included in the optical cavity 43 is kept
constant.

[0086] The VCSEL 41 according to the second exemplary embodiment has the
foregoing structure. The light-emitting operation of the VCSEL 41 is the
same as that of the VCSEL 1 according to the first exemplary embodiment.
The VCSEL 41 has the same effect as in the first exemplary embodiment.
Furthermore, in the second exemplary embodiment, the auxiliary active
layer 47 exhibiting the gain spectrum whose peak wavelength is longer
than the peak wavelength of the gain spectrum of the main active layer 46
is formed, thereby leading to stable operating characteristics even if
the operating temperature is changed from room temperature to a low
temperature. This effect will be specifically described below.

[0087] FIG. 5 illustrates a common gain spectrum of light produced from an
active layer having a quantum-well structure by stimulated emission. The
horizontal axis of the gain spectrum indicates the wavelength (nm). The
vertical axis indicates the gain coefficient (cm.sup.-1). Note that when
the active layer has a multiple-quantum-well structure, the resulting
gain spectrum is given by the superimposition of gain spectra of quantum
wells included in the active layer. The gain coefficient of the gain
spectrum is reduced as the wavelength is changed from the peak wavelength
to shorter wavelengths. At a specific wavelength or less, the gain
coefficient is changed from a positive value to a negative value because
light having the wavelength is absorbed and lost. The gain coefficient is
reduced as the wavelength is changed from the peak wavelength to longer
wavelengths. In this case, the gain coefficient is not changed into a
negative value because light is not absorbed or lost. That is, as the
wavelength is changed from the peak wavelength to longer wavelengths, the
gain coefficient approaches the horizontal axis (zero).

[0088] When the operating temperature of the VCSEL is changed from room
temperature to a low temperature, the gain spectrum of the active layer
shifts entirely to shorter wavelengths and changes similarly in such a
manner that the maximum gain coefficient is increased.

[0089] The resonant wavelength .lamda.0 of light emitted from the VCSEL 41
is determined by the optical length Lo between the upper reflective layer
45 and the lower reflective layer 44 included in the optical cavity 43 of
the VCSEL 41. The coefficient of linear expansion and the refractive
index of each of media constituting the optical cavity 43 are not
significantly changed. So, the resonant wavelength .lamda.0 of light is
not significantly changed. The temperature characteristics of the VCSEL
41 are thus dominated by the temperature dependence of the quantum wells
constituting the main active layer 46 and the auxiliary active layer 47.

[0090] FIG. 6 illustrates the gain spectrum G1R of the VCSEL 41 including
the main active layer 46 and the auxiliary active layer 47 in simulations
at room temperature. The gain spectrum G1R has a profile obtained by the
superimposition of the gain spectrum of the main active layer 46 and the
gain spectrum (not shown) of the auxiliary active layer 47. To
demonstrate the effect of the auxiliary active layer 47 formed in the
VCSEL 41, FIG. 6 also illustrates the gain spectrum G2R of a VCSEL
including the main active layer 46 without the auxiliary active layer 47
in simulations. The simulations were performed under conditions described
below. The peak wavelength of the gain spectrum of the main active layer
46 was set to about 835 nm. The peak wavelength of the gain spectrum of
the auxiliary active layer 47 was set to about 848 nm. The number of
carriers injected into the main active layer 46 was identical to the
number of carriers injected into the auxiliary active layer 47. For
example, the number of carriers was about 2.0.times.10.sup.18
carriers/cm.sup.3. Furthermore, the resonant wavelength .lamda.0 of the
VCSEL 41 is set in a frequency range where the gain coefficient of the
gain spectrum G1R is positive.

[0091] The peak wavelength of the gain spectrum of the auxiliary active
layer 47 is selected in such a manner that the gain coefficient of the
gain spectrum G1R is larger than the gain coefficient of the gain
spectrum G2R in a frequency range corresponding to wavelengths longer
than the peak wavelength of the gain spectrum G2R when the gain spectrum
of the auxiliary active layer 47 is superimposed on the gain spectrum G2R
of the main active layer 46. Wavelengths (frequencies) on the
longer-wavelength side when each of the gain coefficient of the gain
spectrum G1R and the gain coefficient of the gain spectrum G2R is set to,
for example, g (g>0), are set to .lamda.1R and .lamda.2R,
respectively. In the case where the frequency range between the resonant
wavelength .lamda.0 and the wavelength .lamda.1R is set to W1R and where
the frequency range between the resonant wavelength .lamda.0 and the
wavelength .lamda.2R is set to W2R, the frequency range W1R is wider than
the frequency range W2R.

[0092] Next, in the case where the operating temperature of the VCSEL 41
is changed from room temperature (e.g., about 25.degree. C.) to a low
temperature (e.g., about 0.degree. C.), descriptions will be made with
reference to FIG. 7.

[0093] When the operating temperature is changed from room temperature to
a low temperature, the gain spectrum G1R shown in FIG. 6 is shifted to
shorter wavelengths and changed into a gain spectrum G1L. Furthermore,
the gain spectrum G2R is also shifted to shorter wavelengths and changed
into a gain spectrum G2L.

[0094] Wavelengths (frequencies) on the longer-wavelength side when each
of the gain coefficient of the gain spectrum G1L and the gain coefficient
of the gain spectrum G2L is set to, for example, g, are set to .lamda.1L
and .lamda.2L, respectively. The frequency range between the resonant
wavelength .lamda.0 and the wavelength .lamda.1L is set to W1L. The
frequency range between the resonant wavelength .lamda.0 and the
wavelength .lamda.2L is set to W2L.

[0095] In this case, since the gain spectrum G2R is shifted to shorter
wavelengths, the frequency range W2L is narrower than the frequency range
W2R. Meanwhile, the frequency range W1R is wider than the frequency range
W2R. Unlike the frequency range W2L, the frequency range W1L is not
significantly narrow but is comparable to the frequency range W2R even if
the gain spectrum G1R is shifted to shorter wavelengths. It is thus
possible to ensure a gain coefficient profile similar to the gain
spectrum G2R in a frequency range corresponding to wavelengths longer
than the peak wavelength of the gain spectrum G1L even if the operating
temperature of the VCSEL 41 is changed from room temperature to a low
temperature.

[0096] The gain coefficient of the gain spectrum G1L is larger than that
of the gain spectrum G1R in a frequency range corresponding to
wavelengths shorter than the peak wavelength of the gain spectrum G1L. It
is thus possible to ensure a wide frequency range where the gain
coefficient is positive in a wavelength range including the resonant
wavelength .lamda.0 in comparison with the case where the main active
layer 46 is formed without the formation of the auxiliary active layer
47. Hence, the VCSEL 41 operates stably even if the operating temperature
is changed from room temperature to a low temperature. That is, the
formation of the auxiliary active layer 47 complements a reduction in
gain coefficient in a frequency range corresponding to wavelengths longer
than the peak wavelength. Furthermore, an increase in gain coefficient in
a frequency range corresponding to wavelengths shorter than the peak
wavelength allows the VCSEL 41 to operate stably even if the operating
temperature is changed from room temperature to a low temperature.

[0097] In the foregoing second exemplary embodiment, the auxiliary active
layer 47 is formed at one position below the main active layer 46. Like
the first exemplary embodiment, auxiliary active layers can be formed at
positions above and below the main active layer. Alternatively, an
auxiliary active layer can be formed at one position above the main
active layer.

[0098] In the foregoing second exemplary embodiment, the auxiliary active
layer 47 has a single-quantum-well structure. Alternatively, the
auxiliary active layer 47 can have a multiple-quantum-well structure.

[0099] A third exemplary embodiment will be described below with reference
to FIG. 8. A first feature of the third exemplary embodiment is the
formation of an auxiliary active layer located close to one of an upper
reflective layer and a lower reflective layer or is the formation of
auxiliary active layers located close to both of an upper reflective
layer and a lower reflective layer included in an optical cavity of a
VCSEL. A second feature is the fact that the peak wavelength of the gain
spectrum of the auxiliary active layer is set to a wavelength shorter
than the peak wavelength of the gain spectrum of a main active layer in
order to improve operation at high temperatures. In the third exemplary
embodiment illustrated in FIG. 8, descriptions will be made by taking the
case where the auxiliary reflective layer is formed close to the lower
reflective layer as an example.

[0100] A VCSEL 61 according to the third exemplary embodiment has
substantially the same structure as the VCSEL 41 according to the second
exemplary embodiment. That is, the VCSEL 61 includes an optical cavity 63
on the upper surface of a substrate 62. The optical cavity 63 includes a
lower reflective layer 64, a barrier layer 68, an auxiliary active layer
67 (second active layer), a first cladding layer 69, a main active layer
66 (first active layer), a second cladding layer 70, and an upper
reflective layer 65 stacked in that order. An n-type electrode 71 is
formed on the lower surface of the substrate 62. A p-type electrode 72
having an opening 72A is formed on the upper surface of the upper
reflective layer 65.

[0101] The main active layer 66 is arranged in the middle portion of the
optical cavity 63 in the thickness direction. Like the main active layer
46 according to the second exemplary embodiment, the main active layer 66
has a multiple-quantum-well structure that includes, for example, three
well sublayers 66A, 66B, and 66C functioning as quantum wells. A barrier
layer 66D is formed between the well layers 66A and 66B. A barrier layer
66E is formed between the well sublayers 66B and 66C.

[0102] The auxiliary active layer 67 is arranged in the vicinity of the
lower reflective layer 64 with the barrier layer 68. Like the auxiliary
active layer 47 according to the second exemplary embodiment, the
auxiliary active layer 67 has a single-quantum-well structure.

[0103] Unlike the main active layer 46 and the auxiliary active layer 47
according to the second exemplary embodiment, the peak wavelength of the
gain spectrum of the auxiliary active layer 67 is shorter than the peak
wavelength of the gain spectrum of the main active layer 66.
Specifically, the peak wavelength of the gain spectrum of the main active
layer 66 is set to, for example, about 848 nm. The peak wavelength of the
gain spectrum of the auxiliary active layer 67 is set to, for example,
about 835 nm.

[0104] Like the first cladding layer 49 according to the second exemplary
embodiment, the physical (effective) length of the first cladding layer
69 in the thickness direction is reduced by a value corresponding to, for
example, the auxiliary active layer 67 with respect to the physical
(effective) length of the second cladding layer 70. In this way, the
optical length of the first cladding layer 69 is adjusted by a reduction
method to offset the optical length of the auxiliary active layer 67, so
that the optical length Lo between the lower reflective layer 64 and the
upper reflective layer 65 included in the optical cavity 63 is kept
constant.

[0105] The VCSEL 61 according to the third exemplary embodiment has the
foregoing structure. The light-emitting operation of the VCSEL 61 is the
same as that of the VCSEL 1 according to the first exemplary embodiment.
The VCSEL 61 has the same effect as in the first exemplary embodiment.
Furthermore, in the third exemplary embodiment, the auxiliary active
layer 67 exhibiting the gain spectrum whose peak wavelength is shorter
than the peak wavelength of the gain spectrum of the main active layer 66
is formed, thereby leading to stable operating characteristics even if
the operating temperature is changed from room temperature to a high
temperature. This effect will be specifically described below.

[0106] As illustrated in FIG. 5, when the operating temperature of the
VCSEL is changed from room temperature to a high temperature, the gain
spectrum of the active layer shifts entirely to longer wavelengths and
changes similarly in such a manner that the maximum gain coefficient is
reduced.

[0107] The resonant wavelength .lamda.0 of light emitted from the VCSEL 61
is determined by the optical length Lo between the upper reflective layer
65 and the lower reflective layer 64 included in the optical cavity 63 of
the VCSEL 61. The coefficient of linear expansion and the refractive
index of each of media constituting the optical cavity 63 are not
significantly changed. So, the resonant wavelength .lamda.0 of light is
not significantly changed. The temperature characteristics of the VCSEL
61 are thus dominated by the temperature dependence of the quantum wells
constituting the main active layer 66 and the auxiliary active layer 67.

[0108] FIG. 9 illustrates the gain spectrum G3R of the VCSEL 61 including
the main active layer 66 and the auxiliary active layer 67 in simulations
at room temperature. The gain spectrum G3R has a profile obtained by the
superimposition of the gain spectrum of the main active layer 66 and the
gain spectrum (not shown) of the auxiliary active layer 67. To
demonstrate the effect of the auxiliary active layer 67 formed in the
VCSEL 61, FIG. 8 also illustrates the gain spectrum G4R of a VCSEL
including the main active layer 66 without the auxiliary active layer 67
in simulations. The simulations were performed under conditions described
below. The peak wavelength of the gain spectrum of the main active layer
66 was set to about 845 nm. The peak wavelength of the gain spectrum of
the auxiliary active layer 67 was set to about 835 nm. The number of
carriers injected into the main active layer 66 was identical to the
number of carriers injected into the auxiliary active layer 67. For
example, the number of carriers was about 2.0.times.10.sup.18
carriers/cm.sup.3. Furthermore, the resonant wavelength .lamda.0 of the
VCSEL 61 is set in a frequency range where the gain coefficient of the
gain spectrum G3R is positive.

[0109] The peak wavelength of the gain spectrum of the auxiliary active
layer 67 is selected in such a manner that the gain coefficient of the
gain spectrum G3R is larger than the gain coefficient of the gain
spectrum G4R in a frequency range corresponding to wavelengths shorter
than the peak wavelength of the gain spectrum G4R when the gain spectrum
of the auxiliary active layer 67 is superimposed on the gain spectrum G4R
of the main active layer 66. Wavelengths (frequencies) on the
shorter-wavelength side when each of the gain coefficient of the gain
spectrum G3R and the gain coefficient of the gain spectrum G4R is zero
are set to .lamda.3R and .lamda.4R, respectively. In the case where the
frequency range between the resonant wavelength .lamda.0 and the
wavelength .lamda.3R is set to W3R and where the frequency range between
the resonant wavelength .lamda.0 and the wavelength .lamda.4R is set to
W4R, the frequency range W3R is wider than the frequency range W4R.

[0110] Next, in the case where the operating temperature of the VCSEL 61
is changed from room temperature (e.g., about 25.degree. C.) to a high
temperature (e.g., about 50.degree. C.), descriptions will be made with
reference to FIG. 10.

[0111] When the operating temperature is changed from room temperature to
a high temperature, the gain spectrum G3R is shifted to longer
wavelengths and changed into a gain spectrum G3H. Furthermore, the gain
spectrum G4R is also shifted to longer wavelengths and changed into a
gain spectrum G4H.

[0112] Wavelengths (frequencies) on the shorter-wavelength side when each
of the gain coefficient of the gain spectrum G3H and the gain coefficient
of the gain spectrum G4H is zero are set to .lamda.3H and .lamda.4H,
respectively. The frequency range between the resonant wavelength
.lamda.0 and the wavelength .lamda.3H is set to W3H. The frequency range
between the resonant wavelength .lamda.0 and the wavelength .lamda.4H is
set to W4H.

[0113] In this case, since the gain spectrum G4R is shifted to longer
wavelengths, the frequency range W4H is narrower than the frequency range
W4R, which is a very narrow range. Meanwhile, the frequency range W3R is
wider than the frequency range W4R. Unlike the frequency range W4H, even
if the gain spectrum G3R is shifted to longer wavelengths, the frequency
range W3H is not significantly narrow but has a certain width. It is thus
possible to ensure a certain gain coefficient in a range around the
resonant wavelength .lamda.0 even if the operating temperature of the
VCSEL 61 is changed from room temperature to a high temperature.

[0114] In a frequency range corresponding to wavelengths longer than the
peak wavelength of the gain spectrum G3H, the gain coefficient of the
gain spectrum G3H is lower than that of the gain spectrum G3R but is
closer to the gain coefficient of the gain spectrum G4R. It is thus
possible to ensure a wide frequency range where the gain coefficient is
positive in a wavelength range including the resonant wavelength .lamda.0
in comparison with the case where the main active layer 66 is formed
without the formation of the auxiliary active layer 67. Hence, the VCSEL
61 operates stably even if the operating temperature is changed from room
temperature to a high temperature. That is, the formation of the
auxiliary active layer 67 complements a reduction in gain coefficient in
a frequency range corresponding to wavelengths shorter than the peak
wavelength, suppresses a reduction in gain coefficient in a frequency
range corresponding to wavelengths longer than the peak wavelength, and
allows the VCSEL 61 to operate stably even if the operating temperature
is changed from room temperature to a high temperature.

[0115] In the foregoing third exemplary embodiment, the auxiliary active
layer 67 is formed at one position below the main active layer 66. Like
the first embodiment, auxiliary active layers can be formed at positions
above and below the main active layer. Alternatively, an auxiliary active
layer can be formed at one position above the main active layer.

[0116] In the foregoing third exemplary embodiment, the auxiliary active
layer 67 has a single-quantum-well structure. Alternatively, the
auxiliary active layer 67 can have a multiple-quantum-well structure.

[0117] In the second and third exemplary embodiments, the reduction method
includes forming the first cladding layer 49 and the first cladding layer
69 in such a manner that each of the first cladding layer 49 and the
first cladding layer 69 has a short physical (effective) length. However,
the present invention is not limited to this reduction method. As another
reduction method, for example, the first cladding layer 49 and the first
cladding layer 69 can be formed, without changing their physical
(effective) lengths, using a material having a lower refractive index
than that of the cladding layer used when the auxiliary active layer 47
and the auxiliary active layer 67 are not formed.

[0118] In the foregoing exemplary embodiments, the descriptions are made
by taking the 0.8-.mu.m-band VCSELs 1, 41, and 61 as examples.
Embodiments of the present invention can be applied to surface emitting
devices that emit light having longer wavelengths than light from the
VCSELs described above. Alternatively, embodiments of the present
invention can be applied to surface emitting devices that emit light
having shorter wavelengths than light from the VCSELs described above.

[0119] In the foregoing exemplary embodiments, each of the main active
layers 6, 46, and 66 has a multiple-quantum-well structure including the
three well sublayers 6A to 6C, 46A to 46C, or 66A to 66C. A
multiple-quantum-well structure including two or four or more well
sublayers can be used. Alternatively, a single-quantum-well structure can
be used.

[0120] In the foregoing exemplary embodiments, the optical lengths Lo
between the lower reflective layers 4, 44, and 64 and the upper
reflective layers 5, 45, and 65 included in the optical cavities 3, 43,
and 63 are set to values equal to the wavelengths .lamda. of light
emitted, i.e., one wavelength. However, the present invention is not
limited thereto. For example, like Japanese Unexamined Patent Application
Publication No. 2007-87994, the optical length between the lower
reflective layer and the upper reflective layer may be set to a value 1.5
or more times the wavelength .lamda. of light or a value (e.g.,
1.5.lamda., 2.lamda., or 2.5.lamda.). In this case, a main active layer
(first active layer) is formed, and one or more auxiliary active layers
(second active layers) are formed close to one or both of an upper
reflective layer and a lower reflective layer included in an optical
cavity, the main active layer and the one or more auxiliary active layers
being located at plural antinodes of a standing wave.

[0121] In the foregoing exemplary embodiments, the descriptions of the
surface emitting devices are made by taking the VCSELs 1, 41, and 61 as
examples. Alternatively, embodiments of the present invention can be
applied to surface emitting diodes (LED) serving as surface emitting
devices.

[0122] While preferred embodiments of the invention have been described
above, it is to be understood that variations and modifications will be
apparent to those skilled in the art without departing from the scope and
spirit of the invention. The scope of the invention, therefore, is to be
determined solely by the following claims and their equivalents.